let-7 primary transcripts are first detected at the end of the first larval stage and exhibit pulses of expression near the molt of each larval stage. Both of the
plet-7::GFP constructs drove temporally regulated expression, initially observed at the end of the first larval stage (L1). Starting in late L1 (~10 h at 25°C), robust GFP was seen in muscle and neuronal cells in the head, including the nerve ring. Midway through L2, GFP accumulated in the neurons and muscle cells of the tail, body wall muscle (BWM), and ventral nerve cord (VNC). Beginning in L3, all transgenic lines began to express GFP in vulva precursor cells. After the onset of GFP expression in a particular tissue, it was maintained until adults became gravid with embryos (~52 h at 25° C). GFP was not detected in the germ line. Although
plet-7A::GFP and
plet-7B::GFP transgenics showed similar spatio-temporal expression patterns in most cell types, they were not entirely overlapping. In hypodermal and seam cells, the
plet-7B::GFP reporter produced a robust GFP signal at the L3 stage. However,
plet-7A::GFP animals exhibited only weak, transient expression of GFP in the hypodermal cells beginning at the L3 stage. Early in the second stage of development, all intestinal cells showed high expression of GFP in
plet-7B::GFP transgenic animals, while GFP was undetectable in the intestine of
plet-7A::GFP transgenic animals.